1. Field of the Invention
The present invention relates to a method of coating fibres which can be used for the production of tapes and prepregs, fibre-reinforced plastic granules and fibre-reinforced mouldings. The aim of the invention is to simplify and improve the coating of fibres by the dry coating method.
2. Background of the Related Technology
The production of plastic-coated fibres, for example in the form of strands (rovings, mats and/or woven fabrics), tapes, prepregs or fibre-reinforced plastic granules, is known per se. The conventional procedure is to coat fibres with a plastic, possibly containing various additives, and process them to granules. Such granules are often rod-shaped. They are then processed to fibre-reinforced mouldings by methods known per se, for example by compression.
Fibre-reinforced plastic granules can be produced with chopped glass fibres, for example by a compounding method known per se. In this procedure the fibres are shortened by the high shear stress to a residual length of less than 1 mm. Compared with granules reinforced with long fibres, the mouldings from granules produced by the compounding method normally have low strength values.
Another known method is hot-melt coating for the production of thermoplastics reinforced with long fibres. In this wet impregnation method the fibre strands to be coated are prepared in the form of rovings or braids (mats and/or woven fabric). These are passed through a melt containing thermoplastics or thermosets, left to cool and then optionally granulated. However, in the practical application of this method, especially when the proportion of fibres is high and their length increases, the strength values are found to have a high scatter and the moulding exhibits numerous localized weaknesses.
In another known method the fibre strands to be coated are drawn through a liquid impregnating bath containing the plastic, for example a resin, dissolved in a solvent, the appropriate curing agent and an accelerator, together with additives. After the fibres have been drawn through this impregnating bath, the solvent is removed from the coating by evaporation. The strand is then granulated. This method has the disadvantage of using a solvent. Furthermore, the production rate of the coated fibre braids is comparatively low because the solvent has to be evaporated off and removed. If the coating operation is carried out with a reaction resin, for example an epoxy resin, the coated fibre is not stable on storage at room temperature.
In another known method of producing granules reinforced with long fibres, the fibre strands to be coated are moved by a dry method through a fluidized bed consisting of a powdered plastic already incorporating all the additives, this powder attaching itself to the fibres in the form of a coating. The now coated fibres are then heated and the plastic attached to the fibre is liquefied. The coated fibres are cooled and then granulated. If a reaction resin mixture is used, the coated material obtained is precrosslinked in the heating process. These granules are then processed to fibre-reinforced mouldings, preferably by compression. The coated fibres and granules obtained by this method using reaction resins, for example epoxy resins, have the disadvantage that they are not stable on storage at room temperature and that the flow properties of such products constantly change. The low latency of such moulding compounds constitutes a further problem by setting narrow limits on the productivity of the compounding method and considerably restricting the possibility of reactive adjustment of the moulding compounds for coating.